Diagnosing entanglement in quantum materials out of equilibrium would be particularly impactful, as ultrafast lasers have led to the synthesis of nontrivial states of matter without equilibrium analogues [1]}, [2]}, [3]}. Dynamical entanglement has been theoretically demonstrated in the wavefunction of solvable toy models [4]}, [5]}, [6]}, but so far it has been unclear how to experimentally characterize this phenomenon in pump-probe spectroscopic experiments. Inspired by quantum metrology, entanglement can be quantified by the fluctuations of local probes at a given distance or a finite momentum, and the recently developed technique of time-resolved resonant inelastic x-ray scattering (trRIXS) technique [7]}, [8]}, [9]} — sensitive to collective charge, orbital, spin, and lattice excitations [7]}, [11]}, [12]}, [13]}, [14]} — opens new opportunities for diagnosing entanglement in light-driven systems.
